Method and apparatus for surface mounted power transistor with heat sink

ABSTRACT

A surface mounted power transistor is provided with a heat sink by positioning a mounting plate of a heat sink between the power transistor and a solder pad on the circuit board. The mounting plate of the heat sink is provided with a plurality of openings through which the solder of the solder pad flows during the solder reflow process so that the mounting plate is securely adhered between the power transistor and the circuit board. The mounting plate of the heat sink is connected thermally to an extension member which extends generally perpendicular to the mounting plate, the extension member in turn being connected to a heat dissipation surface which may be one or several fins.

CROSS-REFERENCED TO RELATED CO-PENDING APPLICATION

This application claims the benefit of U.S. Provisional Application No.60/173,873, filed Dec. 30, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a method and apparatus formounting a heat sink on a heat producing electrical circuit component.

2. Description of the Related Art

Heat producing components such as field effect transistors often requireheat sinks to carry away the thermal energy produced by the component.To increase the heat dissipation of the heat sink by positioning theheat sink in an air flow, typical mounting arrangements for these heatproducing components include providing a finned heat sink that iselevated off a mounting surface or circuit board to which the combinedheat sink/component is mounted, and the heat producing component itselfis typically mounted on the elevated surface of the heat sink instead ofdirectly on the mounting surface. For field effect power transistors,the transistor may instead be mounted in a vertical orientation on acircuit board with the heat sink mounted to one side of the transistor,thereby positioning the heat sink in an air flow. In anotherarrangement, the field effect transistor is mounted horizontally on thecircuit board or mounting surface and a heat sink is mounted on theopposite surface of the field effect transistor to position the heatsink in the air flow or the heat sink is mounted on an opposite surfaceof the circuit board from the field effect transistor.

Each of these methods for mounting a heat sink to a heat producingcomponent requires a separate mounting step for the mounting of the heatsink from the step mounting the component, each requires a separateaffixing location for the heat sink and the component, and each resultsin a relatively high profile structure.

SUMMARY OF THE INVENTION

An objective of the present invention is to provide a heat sink for asurface mountable heat producing component which affixes the surfacemountable component to the circuit board and the heat sink to thesurface mountable component in a single step. Another objective is toprovide a low profile heat sink. A further objective is to provide aheat sink having a small foot print occupying a minimum area on amounting surface. Yet a further objective is the invention is tominimize space interference between the heat sink and other componentsto be mounted on the mounting surface. Still another objective of theinvention is to provide a surface mountable heat sink.

The invention provides the advantage of eliminating the need for aseparate mounting step of a heat sink while permitting low profilesurface mounting of the heat producing component, all without requiringmounting holes through the circuit board. The present invention alsoprovides a low profile heat sink, enabling more compact circuits to beproduced with tighter clearances between adjacent circuit boards, forexample. The present invention also eliminates the need for multipleaffixing locations by requiring application of solder only to a singlesurface for mounting of the component to a circuit board and formounting of the heat sink.

These and other objectives and advantages of the invention are providedby a heat sink having a mounting plate which extends between a mountingsurface of the heat producing component and the circuit board on whichthe component is to be mounted. The mounting plate of the heat sink isprovided with solder conducting openings which pass through the plate. Asolder pad is formed on the circuit board or on the heat producingcomponent and the mounting pad is positioned against the solder pad sothat during a solder reflow process, the liquid solder flows through thesolder conducting openings to solder the mounting plate of the heat sinkto the circuit board and to solder the heat producing component to themounting plate of the heat sink.

The mounting plate of the heat sink is thermally connected to a heatdissipating element. The heat dissipating element may be a traditionalfinned heat sink structure. However, various preferred embodiments ofthe invention provide heat dissipating elements which present a lowprofile. In particular, the heat dissipating element in each of thepreferred embodiments extends substantially parallel to the circuitboard on which the component is mounted, and so provide a low profilefor the component and heat sink combination from the surface of thecircuit board. Miniaturization of the circuit is thereby facilitated.

For purposes of the present invention, the term heat producing componentrelates to any component to be cooled, and references to circuit boardsor mounting locations refer to any location or surface at which thecomponent is to be mounted.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a power transistor for surfacemounting on a circuit board between which is provided the mounting plateof the present heat sink;

FIG. 2 is a perspective view of a first embodiment of the present heatsink according to the principles of the present invention;

FIG. 3 is a perspective view of a second embodiment of the present heatsink;

FIG. 4 is a perspective view of a third embodiment of the heat sink ofthe invention; and

FIG. 5 is a perspective view of a fourth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, a power transistor 10 has a housing 12 and three surfacemount leads 14 extending from the housing 12. The power transistor 10 isa field effect transistor (FET), although for purposes of the presentinvention it may be any heat producing component or component to becooled. The power transistor 10 is to be mounted on a circuit board 16.In the illustration, the power transistor 10 is to be surface mountedonto the circuit board 16 by soldering of the transistor leads 14 ontosolder pads 18 on the circuit board. The solder pads 18 are connected bysurface leads or lands 20 to other components (not shown) on the circuitboard 16 which together with the power transistor form an electricalcircuit. The power transistor 10 is to be surface mounted, so that it ismounted flat, lying substantially parallel to the circuit board 16. Notonly does this facilitate the surface mounting of the transistor leads14, but it also provides a lower clearance for the component above thecircuit board 16 to facilitate miniaturization of the electronic devicein which the component is used. The power transistor 10 has a metalplate 15 on its mounting surface, and the metal plate 15 has a tab 17extending from the housing along one edge thereof. In the illustration,the tab 17 extends from the edge opposite the leads 14, although it mayextend from another edge.

According to the present invention, a mounting plate 22 is positionedbetween the circuit board 16 and the power transistor 10. In theillustrated embodiment, the mounting plate 22 has the same profile shapeas the mounting surface of the power transistor housing 12.Specifically, the mounting plate 22 has the same shape at the metalplate 15. The mounting plate 22 has a plurality of openings 24 that, inthe illustrated example, are in a regular arrangement in the mountingplate.

A vertical portion or extension 28 extends perpendicularly from themounting plate 22. The extension 28 has a channel 27 that is formed toaccept the tab 17. The tab 17 of the power transistor 10 is insertedinto the channel 27 and the channel 27 is crimped closed to hold thetransistor 10 in position until completion of the mounting process. Thiscrimping of the channel 27 on the tab 17 is referred to as apre-mounting of the power transistor to the mounting plate 22.

The extension 28 extends to a heat dissipation element (see FIG. 2, forexample) which may be any configuration capable of dissipating the heatgenerated by the power transistor 10 during its operation. For example,the heat dissipation element may be a traditional finned structure.Other shapes are possible as well. The mounting plate 22 and theextension 28 along with the heat dissipation element are of a thermallyconductive material, such as copper. It is preferred that the mountingpad and heat dissipation element be shaped so that they may be formed byextrusion.

On the surface of the circuit board 16 is a solder pad 26 for solderingthe mounting pad 22 and power transistor 10 to the circuit board 16.Although the solder pad 26 may be generally of the same surface area anddimensions of the mounting plate 22, it is preferably slightly larger inarea than the mounting pad 22 so as to increase the quantity of solderin the to solder pad 26. The solder pad 26 may instead or in addition bethicker than is required to solder only the power transistor 10 inplace.

A ground connection for the power transistor 10 is provided by a groundlead 29 that is on the surface of the circuit board 16 and which extendsunder the solder pad 26.

During assembly, the circuit board 16 is prepared by forming the circuitleads 20 and the ground lead 29 at their respective locations forconnection into the electrical circuit. The solder pads 18 and the largesolder pad 26 is also formed on the circuit board 16. Solder is printedonto the circuit board 16 according to the known solder applicationtechniques. For example, a mask having openings for the solder pads 18and 26 is positioned over the circuit board 16. A squeegee or a doctorblade is used to spread the solder material evenly and with a uniformlyflat surface. The mask has an opening larger than the mounting plate 22to form the solder pad 26 in a slightly larger area than the mountingplate 22. Alternately, by shaping the mask to be thicker in the regionof the solder pad 26, a thicker application of solder at the solder pad26 is accomplished as a way to increase the quantity of solder in thesolder pad 26.

After the circuit board structures have been formed, the mounting plate22 with the power transistor 10 crimped in the channel 27 to retain thetwo in position is placed atop the solder pad 26. The transistor leads14 are aligned with and positioned atop the circuit solder pads 16.Following the positioning step, a solder reflow process is undertaken byheating the components and the circuit board, such as in an oven. Thesolder pad 26 liquifies and some of the solder from the pad flowsthrough the openings 24 to the mounting surface at the metal plate 15 atthe underside of the power transistor 10 and some of the solder remainsbetween the circuit board 16 and the mounting plate 22, so that whencooled the solder affixes the power transistor 10 to the mounting plate22 which is in turn affixed to the circuit board 16 by the solder. Sincesome of the solder of the solder pad is sucked up through the openings24, the solder pad whish is slightly larger than the mounting plateprovides the extra quantity of solder needed.

The transistor leads 14 are also soldered to the circuit solder pads 18during this solder reflow process. Any other solder connections forother components on the circuit board are formed simultaneously duringthis reflow step. Thus, the mounting of the power transistor 10 to thecircuit board 16 for electrical operation simultaneously mounts themounting plate 22 of the heat dissipation sink in place.

In the preferred embodiment, the mounting plate 22 and its associatedheat dissipation element are of copper and the solder conductingopenings 24 are generally of 0.040 inches in diameter. This size openinghas been found to readily conduct the liquified solder through themounting plate 22 to the transistor surface during the reflow process.Other sizes of solder conducting openings 24 are contemplated as neededfor solder or adhesive materials of different viscosities. An adhesive,such as a thermally activated adhesive material may be provided in placeof the solder pad 26. The solder conducting openings 24 are arranged ina grid arrangement. The openings may be in some other arrangement,including staggered rows, for example. The openings 24 are circularbores but may be slots, channels, or other shapes as desired.

Only a single reflow operation need be performed to mount and bond allcomponents together in a single step. The resulting assembly is of a lowprofile and a small size, and assembly is rapid since separate fasteningsteps are eliminated. The size of the mounting plate 22 and the solderpad 26 there underneath can be configured to conform to any transistoror other heat generating component requiring cooling.

In FIG. 2 is shown an illustration of a heat conducting element 30 whichextends from the vertical extension 28 to a position generally parallelto and overlying the mounting plate 22. The heat dissipation element 30is spaced from and lying over the power transistor 10 when in place sothat the resulting heat sink assembly occupies no greater surface areaof the circuit board than the power transistor itself and only slightlymore height from the circuit board 16 than the transistor alone. Theheat dissipating element 30 is a plate formed by bending a sheet ofthermally conductive material at two parallel lines into a generally “U”shape, one leg of the “U” being the mounting plate 22 with openings 24.The heat dissipating element 30 is approximately the same size as themounting plate 22 as seen in FIG. 2, although other sizes of the heatconducting element 30 are also contemplated. To obtain sufficient heatdissipation surface for most applications, the heat dissipating elementis at least as large as the transistor 10.

An alternative embodiment of the heat sink is shown in FIG. 3 wherein aheat dissipation element 32 is provided over and generally parallel tothe mounting pad 22 but extending to a greater length in a transversedirection than the embodiment of FIG. 2. The heat sink of FIG. 3provides a greater thermal dissipation surface than the embodiment ofFIG. 2. However, the element 32 has lateral extensions 33 that mayinterfere with some circuit arrangements.

Should the circuit components on the circuit board 16 require a heatdissipation element arranged differently then shown in FIG. 3, analternative arrangement of the heat dissipation element 34 is shown inFIG. 4 wherein the heat dissipation element 34 is bent in a directionopposite that shown in FIG. 3 so that from a side view the device has aZ-shape. The element 34 has lateral extensions 35 at each end. The heatdissipation element 34 and extensions 35 extend over a region of thecircuit board 16 at a position adjacent one side of the transistor orother element 10. Some circuit board arrangements may better utilize theembodiment of FIG. 4 rather than FIG. 3, or visa versa. Both are formedfrom the same blank by bending in a different direction.

In the embodiments of FIGS. 3 and 4, the extended portions 33 and 35 ofthe heat dissipation elements 32 and 34 are in a traverse directionrelative to the vertical extension 28 from the power transistor. Shouldthe components on the circuit board onto which the device is mountedlack clearance for such transverse extensions, a sink heat as shown inFIG. 5 may be provided. In the embodiment of FIG. 5, an extended heatdissipation element 36 extends both over the mounting pad 22 and in anopposite direction relative to the vertical extension 28. The heatdissipation element 36 may be formed from a flat blank by bendingportions back on themselves to achieve the shape of FIG. 5. The heatsink of FIG. 5 is formed alternatively by extrusion. It may also beformed by stamping and folding the sheet material back on itself.

The heat sink is shown in the present invention is of a simpleconstruction which may be formed from a sheet of thermally conductivematerial such as copper. The embodiments of FIGS. 2, 3 and 4 are allformed by merely cutting, drilling and bending the sheet withoutassembly of any further parts. In FIG. 5, the heat dissipation surface36 would require an assembly step or an extrusion to form thisembodiment. It is contemplated that many other arrangements of heatdissipation elements such as multiple finned surface or the like may beprovided to carry the heat from the heat generating component 10,through the mounting pad 22, through the vertical extension 28 and tothe heat dissipating element 30-36. Heat is, of course, dissipated fromthe pad 22 and extension 28 as well as from the component 10 itself.Greater heat dissipation may be achieved by heat dissipating elementswith a greater height from the circuit board. Use of these is acceptableif clearance is not an issue. However, for minimum height requirements,the illustrated embodiments provide heat dissipation with only slightlygreater heights from the circuit board surface than the surface mounttransistor alone.

The heat dissipating element of the illustrated embodiments may be usedas an attachment surface for more conventional heat sink structureswhere space permits and where heat generation requires it. Suchconventional heat sink structures include finned structures. As analternative, the traditional heat sink structure may be formedintegrally with the top portion of the present heat sink. The result isa traditional heat sink with the mounting plate according to FIG. 1.

Thus, there is shown and described a heat sink for a surface mountedcomponent and method which provides for assembly of the heat sink withthe component while mounting the component on the circuit board all inone step. The resulting assembly is compact to provide for a reducedsize for the overall apparatus into which it is incorporated. Theassembled circuit board, heat sink and component assembly is affixedtogether as strong or even more strongly than mounting of the componentdirectly on the circuit board. All of these advantages are achievedusing a single reflow process for the assembly.

Although other modifications and changes may be suggested by thoseskilled in the art, it is the intention of the inventors to embodywithin the patent warranted hereon all changes and modifications asreasonably and properly come within the scope of their contribution tothe art.

I claim:
 1. A heat sink assembly, comprising: a circuit board having amounting pad provided with an adhesive material in a mounting region; amounting plate formed of a thermally conductive material and defining aplurality of adhesive flow openings therethrough, said mounting platehaving a first major surface being positioned on said mounting pad ofsaid circuit board; a heat dissipation element thermally connected tosaid mounting plate and being spaced from said circuit board, said heatdissipating element being disposed in a position to receive air flow onboth sides; a heat generating component mounted on said mounting plateat a second major surface opposite said first major surface, said heatdissipating element being spaced from said heat generating component topermit air flow between said heat dissipating element and said heatgenerating component; an extension generally perpendicular to saidmounting plate in a direction opposite said first major surface andextending substantially along the entire length of said mounting plate;and a portion of said extension generally parallel to said mountingplate and spaced therefrom.
 2. A heat sink assembly as claimed in claim1, wherein said adhesive material is electrical solder.
 3. A heat sinkassembly as claimed in claim 1, wherein said adhesive material isthermal adhesive.
 4. A heat sink assembly as claimed in claim 1, whereinsaid portion overlies said mounting plate.
 5. A heat sink as claimed inclaim 1, wherein said portion includes lateral extensions.
 6. A heatsink as claimed in claim 1, wherein said mounting plate and saidextension and said portion form a U shape.
 7. A heat sink assembly asclaimed in claim 1, wherein said circuit board is in a first plane, saidheat dissipating element is in a second plane and said heat generatingelement is in a third plane, said first and second planes are spacedapart and said third plane having said heat generating element isdisposed between said first and second plane.
 8. A heat sink as claimedin claim 7, wherein said first and second and third planes aresubstantially parallel to one another.
 9. A heat sink assembly,comprising: a circuit board having a mounting pad provided with anadhesive material in a mounting region; a mounting plate formed of athermally conductive material and defining a plurality of adhesive flowopenings therethrough, said mounting plate having a first major surfacebeing positioned on said mounting pad of said circuit board; a heatdissipation element thermally connected to said mounting plate and beingspaced from said circuit board, said heat dissipating element beingdisposed in a position to receive air flow on both sides; a heatgenerating component mounted on said mounting plate at a second majorsurface opposite said first major surface, said heat dissipating elementbeing spaced from said heat generating component to permit air flowbetween said heat dissipating element and said heat generatingcomponent; and an extension generally perpendicular to said mountingplate in a direction opposite said first major surface and extendingsubstantially along the entire length of said mounting plate, whereinsaid mounting plate and said extension and a portion of said extensionform a Z shape.
 10. A heat sink assembly, comprising: a circuit boardhaving a mounting pad provided with an adhesive material in a mountingregion, a mounting plate formed of a thermally conductive material anddefining a plurality of adhesive flow openings therethrough, saidmounting plate having a first major surface being positioned on saidmounting pad of said circuit board; a heat dissipation element thermallyconnected to said mounting plate and being spaced from said circuitboard, said heat dissipating element being disposed in a position toreceive air flow on both sides; a heat generating component mounted onsaid mounting plate at a second major surface opposite said first majorsurface, and a channel along an edge of said mounting plate, saidchannel receiving a tab extending from said heat generating component.11. A heat sink for a surface mounted heat generating component,comprising: a mounting plate of a generally planer configurationdefining a plurality of openings therethrough for adhesive flow throughsaid openings; an extension member extending generally perpendicular tosaid mounting plate and extending substantially along the entire lengthof said mounting plate; a portion of said extension member generallyparallel to said mounting plate and spaced therefrom; and a heatdissipation element connected to said extension member, said heatdissipation element and said extension member surface and said mountingplate being thermally conductive and said heat dissipating element beingspaced from the heat generating component in a direction parallel to amajor surface of said heat generating component.
 12. A surface mountableheat sink for a component, comprising: a substantially planar mountingplate having an outer extent substantially a same shape and dimensionsas a footprint of the component, said mounting plate defining openingsextending therethrough; a vertical portion extending at a substantiallyright angle from said mounting plate, said vertical portion having afirst end at said mounting plate and a second end opposite said firstend; and a heat dissipating fin connected to said second end of saidvertical portion, said heat dissipating fin having an extent in adirection substantially parallel to said mounting plate and spacedtherefrom and extending substantially along the entire length of saidmounting plate, said heat dissipating fin being spaced from thecomponent in a direction perpendicular to said mounting plate when thecomponent is mounted on said mounting plate so as to define an air gapbetween said heat dissipating fin and the component.
 13. A surfacemountable heat sink and component, comprising: a substantially planarmounting plate having an outer extent substantially a same shape anddimensions as a footprint of the component, said mounting plate definingopening extending therethrough; a vertical portion extending at asubstantially right angle from said mounting plate, said verticalportion having first end at said mounting plate and a second endopposite said first and; and a heat dissipating fin connected to saidsecond end of said vertical portion, said heat dissipating fin having aextent in a direction substantially parallel to said mounting plate andspace therefrom, said heat dissipating fin being spaced from thecomponent when the component is mounted on said mounting plate, and achannel between said mounting plate and said vertical portion, saidchannel receiving a portion of the component when the component ismounted on said mounting plate.
 14. A power transistor mounting,comprising: a power transistor component having a housing and leads anda back plate, said back plate having an edge portion extending beyondsaid housing; a perforate plate disposed against said back plate of saidpower transistor; a channel connected to said perforate plate, saidchannel engaging said edge portion of said back plate; a verticalportion extending from said channel in a direction substantiallyperpendicular to said perforate plate, said vertical portion having afirst end at said channel and a second end opposite said first end; anda heat dissipating fin connected to said second end of said verticalportion, said heat dissipating fin including a planar part substantiallyparallel to said perforate plate and spaced from said power transistorto receive an air flow; said perforate plate and said channel and saidvertical portion and said heat dissipating fin being formed of a heatconducing material to dissipate heat generated by the power transistor.15. A heat sink assembly, comprising: a circuit board having a mountingpad provided with an adhesive material in a mounting region; a mountingplate formed of a thermally conductive material and defining a pluralityof adhesive flow openings therethrough, said mounting plate having afirst major surface being positioned on said mounting pad of saidcircuit board; an extension generally perpendicular to said mountingplate in a direction opposite said first major surface and extendingsubstantially along the entire length of said mounting plate; a portionof said extension generally parallel to said mounting plate and spacedtherefrom; a heat dissipation element thermally connected to saidmounting plate and being spaced from said circuit board, said heatdissipating element being disposed with an air gap below between saidheat dissipating element and an air gap above said heat dissipatingelement; and a heat generating component mounted on said mounting plateat a second major surface opposite said first major surface.